Many neuroblastoma cell lines express wild type p53 protein which nevertheless remains non-functional. Non-functionality of wild type p53 in neuroblastoma is correlated to the proteins sequestration in the cytoplasm. The purpose of this project is to study the mechanism(s) involved in the cytoplasmic sequestration of p53 in neuroblastoma in order to determine whether the mechansim(s) involved play a more general role in p53 shuttling in and out of the nucleus. To date, we have learned that, in several neuroblastoma cell lines, the cytoplasmic localization of p53 is not affected by blockade of nuclear export with leptomycin, suggesting that cytoplasmic sequestration of p53 in these cell lines is not the consequence of hyperactive nuclear export. We have also observed that p53 insensitivity to leptomycin is correlated with p53 insensitivity to proteasome inhibition. Thus, the cytoplasmically sequestered p53 in these cells is resistant to proteasome degradation. At the same time, we have observed significant association of hdm2, a p53-specific ubiquitin ligase, with the cytoplasmic p53 in these cells, together with p53 ubiquitination. Steady-state and DNA-damage-induced phosphorylation of the p53 in these cells is abnormal and this may be relevant to its cytoplasmic sequestration. We are currently examining whether the nuclear localization signal of p53 in these cells is masked by non-covalent association with another protein(s). - p, , neuroblastoma,